This invention relates to an improvement in metal detectors used in forage harvesters, and more particularly to apparatus for producing a more uniform flux density across the feed path of the harvester, even in regions bounded by ferrous parts.
Bohman Pat. No. 4,433,528 discloses a metal detector located in the lower front feed roll of a forage harvester. The metal detector generates a magnetic field between the lower front feed roll and an upper front feed roll so that a ferrous tramp metal object passing between the feed rolls induces a signal in the metal detector. The metal detector then produces an output signal to stop the feed thus preventing damage to the cutter head in the harvester by the metal object.
While the metal detector arrangement disclosed in the Bohman patent is admirably suited to its intended purpose, and is widely used, it suffers a disadvantage when used in a harvester having ferrous side sheets defining the lateral limits of the crop feed path. These ferrous parts distort the magnetic field generated by the metal detector so that there are regions of reduced magnetic intensity near the side sheets at each end of the feed rolls. If a ferrous metal object of a given size passes through one of these regions there is less likelihood that it will be detected than if its trajectory takes it between the feed rolls nearer the center of the crop feed path.
A primary object of the present invention is to provide a simple, economical solution to this problem, the solution requiring no physical change to the metal detector and only a minimum modification of the harvester side sheets.